Use a P Channel FET... connect it such that the body diode is in the 
same direction as a traditional diode would be, and then connect the 
gate to ground.  You'll need one with appropriate ratings - probably Vgs 
would be the most worrisome, but you would need to look at the whole 
picture.

-forrest

Tobias Gogolin wrote:
> Hi Folks!
> I have some solar cells that almost appear to be made to charge Lion
> batteries from discarded cell phones! I use these battery solar cell combos
> to rig little LED flashlights!
> If I connect the PV directly charging the battery I get almost 40 mA
> charging current, however of course in shade or darkness it discharges the
> battery, so i always use a diode!
> However that bit of voltage drop is critical enough to then only get 12mA
> charge current!
> I have been thinking about using a transistor voltage drop (<.3V) or maybe a
> FET, but I'm not sure I am imagining correctly that the circits I'm thinking
> off would indeed prevent reverse currents!
>
> So if anybody could point me to some well established circuits for this
> purpose I'd be very grateful!
>
>
>   

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